Biology Reference
In-Depth Information
high banks with a wide, flat area just upstream. A minimum amount of construction in this sort of loc-
ation will yield a large, deep pond.
Perhaps an even more remarkable instinct causes beavers to vary a dam's conformation according to
conditions. If the current is slow or the span short, the dam is constructed more or less in a straight line.
However, longer dams where the current is strong are curved, with the convex side pointing upstream.
Of course, this just happens to be the way in which humans engineer big dams!
After building their dam to a satisfactory height, the beaver pair next fabricate a dome-shaped house
called a lodge. Like the dam, the lodge is made of a mixture of sticks and mud, although it's construc-
ted with far less care. The beavers simply heap everything up, starting from the bottom of the pond and
extending the mass for several feet above water level. Then, beginning at a safe depth underwater, they
gnaw and dig their way upward until they're above water level. There they scoop and gnaw out a roomy
chamber.
The size of the lodge depends on the number of beavers in the colony. (More about colony size and
composition later.) Lodges six feet or more above water level aren't uncommon, and in exceptionally
old colonies—forty to fifty years old—lodges have been found as high as twelve to fifteen feet above
water level.
The interior of the lodge may consist of a single, all-purpose chamber, or it may have a feeding
chamber and a connected resting chamber. A good deal of room is required to house a number of
beavers, and the interior of an extremely large lodge may be capable of seating several people at once!
Once the lodge is finished, it provides the beavers with great security. Just as the steel rods in rein-
forced concrete add a great deal of strength, the high ratio of sticks to mud in a beaver lodge makes
it highly resistant to attack. Even in summer, with relatively thin walls surrounding the above-water
chamber, it's very difficult for a predator to break into the lodge, and in that event the beavers can
simply exit into the safety of their pond. In winter, when the mud freezes, the lodge becomes as hard as
iron, impregnable even to the largest, fiercest predators.
Although a beaver lodge is admirably suited for ensuring the survival of its occupants, we shouldn't
regard it as the equivalent of a human dwelling. Whenever he gives a lecture about beavers, Thomas
Decker, a biologist with the Vermont Fish and Wildlife Department, likes to point out that a beaver
lodge doesn't fit our notion of a nice, cozy home. The interior is hot and humid in the summer, smelly
at all seasons of the year, and infested with a variety of undesirable trespassers. Hordes of insects often
abound inside a lodge, water snakes like to crawl into it, and otters may enter from time to time; the
latter, incidentally, will try to kill young beavers up to the age of a month or two. Clearly, this is no
palace by human standards!
It's widely and erroneously believed that beavers include fish in their diet. However, beavers are en-
tirely vegetarian. In summer, they mainly consume a variety of aquatic plants, algae, and some grasses
and ferns. Their principal winter food supply is the inner bark of various species of trees and shrubs.
Aspen, willow, alder, and maple are favorites, although they will eat a variety of other hardwood spe-
